1. Field of the Invention
The subject of the invention is a process for producing defined layers or layer systems of polymers or oligomers with controlled structure on arbitrary solid surfaces, wherein the layer is chemically bonded to the solid surface and applied by means of “living”/controlled free radical reaction. Solid surfaces with oligomer or polymer layers, as well as various compounds containing an anchor group as well as a group from which the polymer growth proceeds in accordance with the ATRP mechanism are a further subject of the invention. Such compounds are termed initiators below.
The present invention relates to a process based on the mechanism of “living”/controlled free radical reaction and polymerisation for chemical modification of arbitrary solid surfaces. In this context the solid can consist of an arbitrary material, can be of a solid or porous nature, can be in finely divided form, can be of natural or synthetic origin, or exhibit a heterogeneous surface structure or surface composition. The physico-mechanical properties of the solid used, such as hardness, ductility, deformability or surface roughness, are also unimportant for the process.
Here the term “surface” relates not only to surfaces in the conventional sense, where, in general, a surface is understood to be the boundary between a solid and a gas or a liquid. The term surface also includes the internal surface of a porous material. Going beyond this, when applied to the surface-modified materials the term surface relates quite generally to arbitrary phase boundaries. Thus, a surface can, for example, also be, the internal surface between two different components within a composite material. Examples of this type are composite materials consisting of a polymer matrix and an inorganic reinforcing agent, polymers filled with dyes or a polymer-metal composite; thus, quite generally, composite materials consisting of a polymer matrix and a functional additive.
Surface properties can be tailored by chemical modification of solid surfaces. On the one hand, a desired quality can be imparted to the surface in this way; on the other hand, the quality of the physical interaction of the surface-modified solid with other substances, the chemical reactivity and the capacity for chemical binding of other substances can be adjusted in a targeted manner.
If layers or layer systems are applied to surfaces, it is possible, in the individual case, so to modify the properties of the original surface that the characteristics of the system as a whole are then determined solely by the coating. Thus, it is possible, for example, to impart the requisite mechanical strength to a composite system by means of a suitable carrier material and, on the other hand, by means of the coating system, to adjust the mechanical, physical and/or chemical properties of the surface to those desired.
2. Description of the Related Art
Various techniques are customary In order to modify solid surfaces by application of polymers. For example, processes in which dissolved polymers are sprayed on, or applied by spin coating, dip coating or in accordance with the Langmuir-Blodgett technique (LB films) are described in the literature. With these processes binding of the polymers at the surface is to a very large extent of an adhesive nature. The process parameters are frequently difficult to control with these processes; moreover, the Langmuir-Blodgett technique in particular can be applied only on planar surfaces and is essentially restricted to amphiphilic molecules or molecules having a rigid chain.
Polymer molecules can also be chemically bonded to solid surfaces by forming a covalent chemical bond at the solid surface via, usually, terminal groups of the polymer molecules (“grafting to”, for example via a condensation reaction). A disadvantage of this process is that the yields of such surface reactions, and thus the graft densities of the polymer molecules on the surface, are generally not very high since polymer molecules that have already been bound impede the approach of further molecules to the surface. Furthermore, the process is restricted to polymers of relatively low molar mass, since it is only with small molecules that there is a sufficiently high probability that the functional group of the polymer molecule is withinin reach of the bonding points on the solid surface and a chemical reaction between the two thus becomes possible.
In order to circumvent the disadvantages associated with “grafting to” processes, in further developed processes the polyreactions for formation of polymers are initiated directly at the solid surface (“grafting from”) [J. Rühe, “Massgeschneiderte Oberflächen” (“Tailored surfaces”), Nachr. Chem. Tech. Lab 42 (1994) 1237]. In this context in the prior art on polymerisation reactions using solid surfaces as starting materials the conventional free radical graft reactions are usually described: conventional initiators, i.e. azo compounds, peroxides and the like, are used to initiate the free radical polymerisation reactions. If such initiators are covalently bonded to solid surfaces in order to initiate graft reactions from here, this is thus associated with the following disadvantage: in the case of symmetrical initiators such as, for example, azo-bis-isobutyronitrile (AiBN) or benzoyl peroxide (BPO), after decomposition one fragment is covalently bonded to the solid surface as initiating radical; the second radical fragment, on the other hand, remains unbound and in turn is able to initiate a polymerisation reaction, which, however, takes place not at the solid surface, but unbound. Therefore, in the case of a polymerisation initiation with the abovementioned conventional initiators non-bound polymer is always also formed in addition to non-bound (sic) polymer.
This situation has lead to the search for an alternative via asymmetric initiators, only the bound radical fragment of which has a reaction-initiating action following decomposition. This is, for example, described in detail in the papers by Rühe et al. [O. Prucker, J. Rühe, Macromolecules 31, 592 (1998); O. Prucker, J. Rühe, Macromolecules 31, 602 (1998)].
In addition, in the case of all free radical polymerisation reactions conventionally initiated hitherto these are subject to the conventional kinetics of free radical polymerisation, i.e. the graft branch length and the termination reactions can be only inadequately controlled and the chain length is subject to the typical chain length distributions of conventional free radical polymerizations [see Brurio Vollmert, Grundriss der Makromolekularen Chemie (Principles of Macromolecular Chemistry), Vol. I, E. Vollmert-Veriag, Karlsruhe, 1979]. Furthermore, the chain ends of the graft branches are no longer reactive after the polymerisation reaction, so that, for example, grafting of a second polymer generation is not possible.
This disadvantage of free radical polymerisation has recently been largely eliminated by a new process. If a free radical polymerisation reaction is carried out in accordance with a “living”/controlled free radical mechanism it is possible to produce defined polymers, the chain length and polydispersity of which can be substantially better controlled than is the case in conventional free radical polymerisation. Since the number of chain terminations in this process is greatly reduced, the term “stable free radical polymerisation” (SFRP) is also employed. This process was further refined by K. Matyjaszewski et al., by the introduction of the “atom transfer radical polymerisation” (ATRP) concept [K. Matyjaszewski, S. Coca, S. Gaynor, Y. Nakagawa, S. M. Jo, “Preparation of Novel Homo- and Copolymers using Atom Transfer Radical Polymerisation”, WO 98/01480]. To date “living”/controlled free radical polymerisations, including in their refinement according to the ATRP-mechanism, have been carried out only in the liquid phase, with or without additional solvent.
In ACS Polym. Preprints [Div. Polym. Chem. (39), 626 (1998)] Craig J. Hawker, et al. describe the synthesis and application of polymers using “living”/free radical polymerisation reactions. The initiators used for the free radical polymerisation are compounds which contain nitroxide groups. These compounds also have terminal trichlorosilyl groups, which can be bound to surfaces of silica gel and silicon wafers by chemical reactions.
In Macromoulecules 1998, 31, 5934 Tsujii, et al. describe controlled graft polymerisations of methyl methacrylate on silicon oxide-containing substrates by combined use of the Langmuir-Blodgett (LB) technique and the ATRP (Atom Transfer Radical Polymerisation) technique 2-(4-chlorosulphonylphenyl)ethyltrimethoxysilane is used as initiator compound. This compound possesses a chlorosulphonyl group as initiator group for the “living”/controlled polymerisation. After applying the monolayer of the abovementioned initiator, which has been compressed at a water/air interface, to a silicon wafer by means of the LB technique, the “living”/controlled free radical polymerisation of methyl methacrylate is carried out from the silicon wafer surface modified in this way.
These processes of the state of the art have the following disadvantages: in the case of the “stable free radical polymerisation” SFRP using nitroxides a thermal polymerisation that proceeds simultaneously and does not proceed from the surface frequently takes place because of the requisite high temperatures of 120 to 130° C. Thus, there are considerable disadvantages associated with the process for the “living”/controlled free radical grafting of solid surfaces, specifically                a) non-bound polymer formed consumes monomer,        b) growing bound and non-bound polymer chains compete for nitroxides and thus influence control of the growing chains,        c) non-bound polymer is present as reaction product alongside polymer-modified solid surfaces.        
According to the publication by Tsujii et al. 2-(4-chlorosulphonylphenyl)-ethyltrimethoxysilane is employed as initiator compound. Chlorosulphonylphenyl groups are known to be highly reactive and in particular susceptible to hydrolysis, so that they are difficult to work with. Compounds which contain such groups, and also surfaces provided with these, are unstable.
Furthermore, the LB method described in this publication can be applied only to planar substrate surfaces, and here of limited size, but not to solid surfaces of any size, shape and composition, and also not to internal surfaces of materials having open porosity. The density of the molecules in the layer can be influenced to only an incomplete extent. Estimation of the degree of polymerisation of the grafted polymer molecules is carried out only indirectly; the grafted polymer molecules themselves are not used for this purpose. Furthermore, it is not stated that the chain ends are capable of further initiation.